The flip side of Peter Gabriel's recent covers album, Scratch My Back, with each covered artist in turn covering a Gabriel song, will be issued as a series of monthly iTunes downloads because those involved have either missed deadlines or are refusing to take part.

Ever since Gabriel announced the release of Scratch My Back, he promised a companion piece – called, naturally, I'll Scratch Yours. Gabriel described the project as a "song swap": for his part, he would record orchestral covers of tracks by Bon Iver, Radiohead, Paul Simon and others, then these same artists would choose and record their favourite Gabriel tunes.

Unfortunately, I'll Scratch Yours has not materialised as Gabriel envisioned. Although various artists agreed to do Gabriel covers, they all missed their deadlines. Scratch My Back was released on its own, and Gabriel will release the other covers as a pair of digital downloads each month.

"We may not get everybody, but I hope we will," Gabriel told the New York Times. The first two songs, featuring Paul Simon's version of Biko, and the Magnetic Fields' take on Not One of Us, are available now, with others following over the coming months.

However, Arcade Fire's publicist told Pitchfork that the band are not covering Games Without Frontiers as originally promised. It is unclear whether they are covering another Gabriel song or if they are even participating in the project at all.

The only artist to officially turn Gabriel down was David Bowie, whose Heroes is covered on Scratch My Back. Instead, the song's co-writer, Brian Eno, is to record something instead.